1. Field of the Invention
The present invention relates to a lighting unit for use in a liquid crystal display device or the like and to a liquid crystal display device using the same.
2. Description of the Related Art
FIG. 11 is a perspective view schematically showing a constitution of main parts of a conventional lighting unit for use in a liquid crystal display device. FIGS. 12 and 13 are explanatory views showing a lighting operation of the lighting unit of FIG. 11, in which FIG. 12 is a sectional view schematically showing the lighting unit and FIG. 13 is a top view showing the lighting unit. As shown in FIGS. 11 to 13, the lighting unit includes a bar-shaped light source 1, a light guiding plate 2 disposed such that the bar-shaped light source 1 is located close to a side surface 2A of the light guiding plate 2, a reflection sheet 4 disposed so as to cover at least a bottom surface 2B of the light guiding plate 2 where scattering patterns 3 (FIG. 12) are formed, and a diffusion sheet 5 disposed along a light emanating surface 2C of the light guiding plate 2. For the purpose of improving luminance as seen from front, such a constitution that a prism sheet having a light converging function is further disposed on a top face of the diffusion sheet 5 is also generally practiced.
As shown in FIG. 12, in the lighting unit, light emitted from the bar-shaped light source 1 is incident on the light guiding plate 2 from the side surface 2A (hereinafter, this face is referred to as a light incident face 2A) of the light guiding plate 2. While traveling inside the light guiding plate 2, the light is scattered by the scattering patterns 3 of the light guiding plate 2 and reflected by the reflection sheet 4 (not shown) appropriately. Then, the light emanates upwardly through the light emanating surface 2C of the light guiding plate 2. Further, this emanating light is diffused by the diffusing sheet 5 (not shown).
As the bar-shaped light source 1, a cold-cathode fluorescent light tube is generally used in the above-described lighting unit, while a hot-cathode fluorescent light tube is used in some large-size lighting units. As shown in FIG. 12, when either one of the fluorescent light tubes is used as the bar-shaped light source 1, electrodes 15 that do not emit light by themselves are respectively disposed at both ends of the bar-shaped light source 1. Therefore, in the bar-shaped light source 1, non-light-emitting portions 1B are respectively formed at both ends of the bar-shaped light source 1, and a light-emitting region (hereinafter, referred to as an effective light-emitting region) 1A is formed in a center portion (a portion of a glass tube 16) sandwiched between the non-light-emitting portions 1B.
In the bar-shaped light source 1, because the non-light-emitting portions 1B are formed at both ends of the bar-shaped light source 1, the effective light-emitting region 1A is shorter in length than the overall length of the bar-shaped light source 1. Furthermore, owing to the non-light-emitting portions 1B formed at both ends of the bar-shaped light source 1, the amount of incident light at both ends of the light incident face 2A is reduced, thereby causing two corner portions X of the light guiding plate 2 that are opposite to the non-light-emitting portions 1B of the bar-shaped light source 1 to be darkened. As a result, as shown in FIG. 13, the luminance of light emanating from the light emanating surface 2C of the light guiding plate 2 becomes low at the corner portions X. As used herein, the corner portion X refers to a portion where the light incident face 2A and a side surface 2D adjacent to the light incident face 2A intersect.
In the light guiding plate 2, as described above, its luminance is reduced at the corner portions X, thereby resulting in non-uniform luminance over the entire light emanating surface 2C of the light guiding plate 2. Although boundaries between darkened portions (hereinafter, referred to as dark portions) 10 of the corner portions X, and the remaining bright portion do not appear clearly, in a plan view, the boundaries appear to be inclined so as to extend outwardly from both ends of the effective light-emitting region 1A as a whole (the dark portions 10 in FIG. 13 is schematically shown).
In order to solve the above problem, there is an approach in which the overall length of the bar-shaped light source 1 is extended so that the effective light-emitting region 1A reaches both ends of the light incident face 2A of the light guiding plate 2. However, in this constitution, since the non-light-emitting portion 1B of the bar-shaped light source 1 protrudes from the light guiding plate 2, the overall length of the bar-shaped light source1 is lengthened, causing the size of the entire lighting unit to be increased. As a result, narrowing of a display panel frame, which is required in the liquid crystal display device, is not achieved.